Communication systems are pervasive in modern society. One of the most common is a wireless communications in the current form of cell phones. Geographic features, natural, as well as, man-made can cause issues with wireless communications. Distance, noise, signal strength, fading signals, multi-path signals are but a few of the issues challenging the wireless communications system designer. Designers must also contend with antenna placement, polarization, possible antenna height restrictions, as well as small transmitters with poor antennas, limited battery power, low Effective Radiated Power (ERP), etc. This presents a problem.
Cellular communications presents additional challenges in addition to those mentioned above because of the multitude of personal handsets, their variation, differing simultaneous communications, and many varying locations.
One approach that has been tried is to just use a medium gain omni-directional (omni) antenna to send and receive in all directions. The gain of the system is limited by the gain of the omni. From a transmission perspective, the omni may not present much of a problem if the system is running maximum Effective Isotropic Radiated Power (EIRP). A possible problem with an omni is that when transmitting equally in all directions, some of the signal can bounce off nearby objects and still be strong by the time they arrive at the receiver. This can create multipath distortion. Multipath can be a major source of poor data reception. Additionally, the polarity of the receiving antenna may not be of the same orientation as the omni (vertical for most omnis), so some signal may not be picked up. From the receive perspective, an omni suffers since it has low to medium gain and it is receiving noise and interference from all directions. So for example, if the signal arriving at the omni is not purely vertical polarity, then some signal is lost to polarity mismatch. This can be as high as a 20 dB loss. This presents a problem.
Yet another approach tries to account for this polarity mismatch by using circular polarization (CP) on transmit and receive. However if on receive, a signal is linearly polarized, then there is a 3 dB loss. If circularly polarized antenna(s) are directional, then they must be combined somehow. Using an isolating combiner, any signal out of phase with the main strongest receiving port will be sent to a termination and be lost. Additionally, the classic combining of several antennas pointed in different directions will bring in noise and interference that is not cancelled out due to phase mismatch. On transmit, the power will be transmitted in all directions in CP wasting all but the wanted direction and wasting another 3 dB if the antenna receiving the signal is linearly polarized. This presents a problem.